If the TV is too low, it’ll be hard to see from where you sit. If it is too high, you'll have to sit with your neck at an uncomfortable angle as you watch it.
Aim to position the middle of your screen about 42 inches off the floor at eye level. This should help you avoid having to contort your neck in order to see the screen.
Use a stud finder to locate the studs in the wall, mark their location with painter's tape or a pencil, measure for the correct height, then drill the pilot holes into the studs.
Consider holding the unit where you plan to hang it to simulate viewing. Doing so at various times of the day will let you see how changing light affects the picture.
The heat generated from a fire can make the TV warmer, which can decrease its usable lifespan. Ash and soot may also get into the television and damage its various components.
Use an in-wall cable management system to run all the power cords and cable connections down through the wall to keep them out of sight, or a cord cover to conceal wires.